smozely With a bit more tinkering around, I have a much more solid answer on adding views and such to the Sketch UI. The reason why onOpenDocument seems to be ignored is because when you get try to get the views from the MSDocument from context, it returns null. The reason is because while the action is indeed called onOpenDocument, the initialization actually finishes milliseconds after that Action finishes, and there isn't any Action for the next stage. For me, I got around that by starting asynchronously waiting for the document to not return null, and then starting initialization the moment document is not null.
When it comes to adding views, it's fairly simple process of getting the InspectorView, which contains the NSStackView that holds everything else. From there you just have to make sure you set the right layout constraints so it displays properly.